1. Field of the Invention
The present invention relates to a development unit for developing latent electrostatic images to visible toner images, and more particularly to a development unit in which a toner is supplied onto a development roller and formed into a thin layer by toner layer thickness regulation means.
2. Discussion of the Background
Conventionally, there is known a method of developing latent electrostatic images by a non-magnetic one-component toner. According to this method, the toner is supplied onto a development roller and formed into a thin layer by toner layer thickness regulation means such as a blade- or roller-shaped member, and the thus formed toner layer is then brought into contact with a latent electrostatic image formed on an electrophotographic photoconductor, whereby the latent electrostatic image is developed to a visible toner image.
In the aforementioned development method, the toner layer thickness regulation member is required to have (i) good toner releasability by which toner is released from the toner layer thickness regulating member, (ii) high resistance to wear, and (iii) toner chargeability by which toner is charged to a desired polarity. With the above requirements taken into consideration, conventional toner layer thickness regulation members comprise a metal such as stainless steel, fluoroplastics, or modified fluoroplastics.
However, the toner releasability of the toner layer thickness regulation member which comprises a metal is so poor that the toner partially adheres to the surface of the toner layer thickness regulation member when the toner layer thickness regulation member is brought into contact with a thin toner layer formed on the development roller. As a result, streaks are formed on the toner thin layer on the development roller, which causes white lines in the developed solid images.
In the case where the toner layer thickness regulation member comprises a fluoroplastic, the wear-resistance thereof is considerably so poor that the life thereof is short, although the toner releasability is good. In addition, this kind of toner layer thickness regulation member itself tends to be charged to a negative polarity, so that it can easily charge the toner layer formed on the development roller to a positive polarity, but it is difficult to charge the toner layer to a negative polarity. Therefore, the above toner layer thickness regulation member cannot be used for both the negatively- and positively-chargeable toners.
When the toner layer thickness regulation member comprises a modified fluoroplastic, which is prepared, for instance, by modifying a fluoroplastic using polyethylene, the wear-resistance and the toner chargeability can be improved to some degree. However, the toner releasability is low, so that toner particles adhere to the surface of the toner layer thickness regulation member.
As disclosed in Japanese Laid-Open Patent Application 57-66442, a blade- or roller-shaped toner layer thickness regulation member comprises a silicone resin, modified silicone resin or silicone oil to control the toner chargeability. In such a triboelectric charging member, however, the wear-resistance and the toner releasability are degraded. In addition, the triboelectric charging member of this kind cannot be used in common with both the positively- and negatively-chargeable toners.
To solve the above-mentioned problems, the inventors of the present invention proposed to use a toner layer thickness regulation member comprising a particular dimethyl silicone rubber. In this case, the toner charging performance of the toner layer thickness regulation member mainly depends on the triboelectric performance of the toner layer regulation member, because the above-mentioned dimethyl silicone rubber is electrically insulating. Therefore, when this toner layer regulating member is used, the charge quantity of the toner varies depending on the kind of toner and the material for the development roller in the development unit. Furthermore, it takes a great deal of time and labor to find an appropriate combination of the toner and the material for the development roller in order to obtain the desired charge quantity of the toner.
As a method of freely controlling the charge quantity of toner, a so-called charge injection method is widely employed. In this case, electric charges are injected into the toner layer formed on a development roller by using a toner layer thickness regulation member comprising a material with a middle to high volume resistivity. For use in practice, finely-divided particles with low resistivity, such as carbon and metallic powder, are dispersed in the aforementioned dimethyl silicone rubber to decrease the volume resistivity of the toner layer thickness regulation member.
However, when the toner layer thickness regulation member shows a middle volume resistivity (from 10.sup.6 to 10.sup.12 .OMEGA..multidot.cm), stable charge injection into the toner layer cannot be performed due to considerable variations in the volume resistivity of the regulation member. On the other hand, when the volume resistivity of the toner layer thickness regulation member is less than 10.sup.6 .OMEGA..multidot.cm by dispersing therein a large quantity of finely-divided particles with low resistivity, it has the shortcoming that the toner releasability decreases.